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Coadsorption of Tri-chain Surfactants and Dodecanol at a Hydrophobic Surface Studied by Sum-Frequency Spectroscopy Malkiat S. Johal, Robert N. Ward,† and Paul B. Davies* Department of Chemistry, UniVersity of Cambridge, Lensfield Road, Cambridge CB2 1EW, England ReceiVed: June 12, 1995; In Final Form: September 28, 1995X
In situ sum-frequency vibrational spectra have been recorded of monolayer films adsorbed onto a planar hydrophobic surface from mixed aqueous solutions of tri-n-alkylsulfotricarballylate (tri-chain) surfactants and dodecanol. The change in composition of the film with concentration is similar for all the tri-chain surfactants investigated. At very low tri-chain concentrations the monolayer spectra arise entirely from dodecanol. As the tri-chain concentration increases, adsorbed dodecanol is gradually displaced by the trichain surfactant. At a concentration of one third of the tri-chain critical micelle concentration (cmc), when the dodecanol coverage is approximately two-thirds of a full monolayer, the tri-chain has the same packing density as in the absence of dodecanol. Above the cmc, only the tri-chain adsorbs significantly which is explained by the uptake of dodecanol in bulk phase micelles. The structure of the tri-chain and dodecanol surfactants in the mixed film is similar to that in the pure tri-chain and dodecanol monolayers, with the tri-chain showing significant conformational disorder in contrast to the highly orientated dodecanol. This points to minimal mixing between the alkyl chains of the dodecanol and tri-chain molecules. The simplest interpretation of this behavior is that the tri-chain molecules are adsorbing individually or as clusters, around which dodecanol molecules are able to pack.
Introduction Mixed surfactant systems often show enhanced properties compared with those of pure surfactant solutions. For instance, mixed surfactants are used as solubilizers in many industrial applications since solubilization by mixtures is often superior to solubilization by the individual surfactants.1,2 While many studies have investigated how mixtures determine interfacial properties3-9 and how the composition of the surface is related to that of the solution, relatively few have given any insight into the structure of the surfactant film adsorbed at the interface.10-12 In this work, sum-frequency spectroscopy (SFS) is used to study the structure of dodecanol coadsorbed with a series of sodium tri-n-alkylsulfotricarballylates, anionic tri-chain surfactants with the structure shown in Figure 1a. The alkyl tail group (R) was varied in the series n-butyl-, n-pentyl-, n-hexyl-, and ethylbenzene. These surfactants are subsequently named as tributyl STC, tripentyl STC, trihexyl STC and triethylbenzene STC for brevity. The surfactants used in this study have completely different structures. Trialkyl STC molecules have a very large hydrophobic moiety and a small charged hydrophilic head group which allows them to form microemulsions without the need for co-surfactants.13 In contrast, the hydrophobic and hydrophilic terminal groups of dodecanol (Figure 1c) are both small and uncharged, which prevents micelle formation and leads to low solubility in water. One might anticipate poor mixing of two such dissimilar species, i.e., competition between the species occurring instead of coadsorption. The purpose of this study is to identify the conditions over which coadsorption occurs and to look for spectroscopic evidence of mixing at the interface. This work extends a recent investigation of the coadsorption of sodium dodecyl sulfate (SDS) and dodecanol using SFS which showed that the coadsorption of dodecanol produced a marked increase in the conformational order of the SDS.4
Figure 1. Structure of (a) sodium tri-n-alkylsulfotricarballylates, where R ) n-butyl-, n-pentyl-, n-hexyl-, and ethylbenzene, (b) trihexyl citrate, and (c) dodecanol.
Sum-frequency spectroscopy makes use of the nonlinear optical phenomenon of sum-frequency generation14 to provide a vibrational spectrum of molecules at an interface. Essentially, the interface is irradiated simultaneously with a fixed frequency pulsed visible laser (ωvis), and a tunable pulsed infrared laser (ωir). Light emitted at the sum frequency (ωvis + ωir) is detected as the infrared laser frequency is scanned.15 Resonant features occur at the frequencies of molecular vibrations that are both infrared and Raman active.16 Since sum-frequency emission only occurs from noncentrosymmetric environments, SFS is highly surface specific.17 Thus, surfactant molecules in solution, which exist either as an isotropic collection of monomers or as centrosymmetric aggregates, do not give resonant sum-frequency emission, whereas surfactant monolayers possessing net orientational order are sum-frequency active. The intensity of the sum-frequency signal reflects both the coverage and orientational order in the monolayer.18 These two characteristics of SFS are used here to investigate how the behavior and structure of the coadsorbed tri-chain surfactants and dodecanol compares with their properties as pure monolayers. Materials and Methods
†
Present address: Exxon Technology Centre, Milton Hill, Abingdon, Oxon, UK OX13 6BB. X Abstract published in AdVance ACS Abstracts, December 1, 1995.
0022-3654/96/20100-0274$12.00/0
The sodium tri-n-alkylsulfotricarballylates were synthesized at Kodak Ltd. by dehydration of citric acid to form aconitic © 1996 American Chemical Society
Coadsorption of Tri-chain Surfactants and Dodecanol acid followed by esterification and sulfonation.19-21 Potentiometric titrations of these tri-chains showed their purities were all 98% within the estimated uncertainty of approximately 2%.13 An uncharged tri-chain, trihexyl citrate (Figure 1b), was synthesized by esterification of citric acid (Aldrich, 99%) using hexyl alcohol (Aldrich, 98%). The infrared and 1H NMR spectra were consistent with the required product structure. The DEI mass spectrum showed an MH+ (“self CI”) pseudomolecular species at m/z 445 and a fragmentation pattern which was consistent with the structure of the product. All solutions were prepared with ultrapure water (Elga Maxima, quality > 18 MΩ cm). Stock solutions of the trichains were prepared at 5 times the critical micelle concentration (cmc), either pure or containing dodecanol (Aldrich, 98%) at 2.1 × 10-3 M. To ensure that the dodecanol was solubilized in the tri-chain micelles, the solutions were sonicated at 50 °C for 30 min. Pure solutions of tri-chains below the cmc were prepared by diluting the stock solutions with water. Mixed solutions of tri-chain and dodecanol were prepared by diluting the stock with pure solutions of the tri-chain so that the dodecanol was present at 1.05 × 10-5 M (the solubility limit in nonmicellar solutions at 18 °C). A saturated aqueous solution of tri-hexyl citrate was prepared by adding 50 µL to 10 cm3 of water and sonicating the solution at 50 °C until a dispersion was formed. The Cambridge nanosecond sum-frequency spectrometer has already been described in detail in the literature.18,22,23 The model hydrophobic substrates were self-assembled monolayers of perdeuterated octadecanethiol (d-ODT) chemisorbed on a thin film of gold that had been evaporated onto chromium primed silicon wafers.24 Spectra were taken in a spectrochemical cell, described elsewhere,18,22 containing the substrate and aqueous surfactant solution. To minimize infrared absorption by water, a thin film (∼1 µm) of the solution was trapped between the substrate and a calcium fluoride prism during acquisition of the SF spectra. The prism allowed entry of the counterpropagating infrared and visible beams onto the sample and exiting of the SF beam to the detector. All three beams were p-polarized. The surfactant solution and the substrate were allowed to equilibrate for 30 min at 18 ( 1 °C before the sample was brought close to the prism. For solutions where the surfactant concentration was below 10-4 M, the time was extended to 1 h. Results Pure Surfactants. It is first necessary to understand the spectral information obtained using pure surfactant solutions before considering mixed systems. Figure 2 shows SF spectra in the C-H stretching region of films from pure aqueous solutions of dodecanol, tributyl STC, tripentyl STC, trihexyl STC, trihexyl citrate, and triethylbenzene STC. In these spectra the solution concentrations of the tri-chains were twice their cmc, and the dodecanol and trihexyl citrate spectra were obtained using saturated solutions. The spectra consist of a nonresonant signal from the gold surface, which is virtually frequency independent, upon which resonant features from the interfacial molecules are superimposed. Since the self-assembled monolayers used were fully deuterated, all their resonances lie at frequencies lower than the IR region investigated. In addition, control experiments were carried out to show that no SF signal arose from molecules adsorbed on the prism surface and that there was insignificant infrared absorption by the solution trapped between the substrate and the prism. If the surfactant were adsorbed to the prism surface, the relative phase of the light emitted from the surfactant and the gold
J. Phys. Chem., Vol. 100, No. 1, 1996 275
Figure 2. Sum-frequency spectra in the C-H stretching region: (a) dodecanol, (b) tributyl STC, (c) tripentyl STC, (d) trihexyl STC, (e) trihexyl citrate, (f) triethybenzene STC. The concentrations of the trialkyl STC surfactants were twice their cmc. Dodecanol and trihexyl citrate were used as saturated solutions. The spectra from (a) to (e) are offset vertically for clarity; zero intensity refers to (c).
surface would vary as the separation of the prism and the gold was changed. The SF spectra of trihexyl STC was found to be independent of the film thickness of the solution and hence the surfactant molecules detected are adsorbed at the self-assembled monolayer substrate. The C-H stretching modes, summarised in Table 1, can be assigned to the modes of aliphatic groups (spectra 2a-e) and of aromatic groups (spectrum 2f). The position and intensity of resonance dips or peaks can be precisely determined using fits to the spectra.16 Considering the aliphatic groups first, the features at 2873 and 2932 cm-1 are assigned to the symmetric methyl stretch (r+) split by a Fermi resonance. The resonance at 2962 cm-1 is assigned to the antisymmetric methyl stretch (r-).22 The features at around 2855 and 2915 cm-1 arise from stretching modes of methylene groups (d+).18 Triethylbenzene STC does not show any of these resonances but does give other features assigned to the aromatic group, the most intense of which lies at 3069 cm-1 and is due to the symmetric stretch of the aromatic C-H group, ν2.25 The features at 2960 and 2920 cm-1 have not hitherto been assigned and may arise from overtones of an asymmetric C-C stretch (ν14 and ν19b).26 In an all-trans hydrocarbon chain, methylene groups are in a locally centrosymmetric environment and hence SF inactive. The methylene resonances are therefore assigned primarily to CH2 groups associated with gauche defects, which reduce the
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TABLE 1: Assignments of Resonant Modes Observed in the Sum-Frequency Spectra of Surfactants Adsorbed to dODT/Au resonant freq (cm-1)
sym stretch split by Fermi resonance (low-freq component) 2932 high-frequency component of Fermi resonance 2962 antisym stretch chain methylene 2851-2858 sym stretch split by (CH2) Fermi resonance (low freq) 2900-2930 high-frequency Fermi resonance terminal phenyl 2920 overtones of an asym C-C (C6H5) stretch 2960 3069 sym stretch of arom CH terminal methyl (CH3)
2873
assignmenta
label r+ r+FR rd+ d+FR V19b V14 V2
a The frequencies of methyl and methylene modes are based on theoretical fitting of the SF spectra.16 The assignments of the phenyl modes have been deduced by comparison with IR spectra of benzene derivatives.25,26
local symmetry. Simple inspection of the methyl (r) and methylene (d) resonances therefore yields important information about the structure of the adsorbed film. The intensity of the methyl modes increases with decreasing population of gauche conformers and vice versa since gauche conformations tend to decrease the orientational order of the methyl groups. The ratio of the methyl and methylene resonant strengths provides a qualitative measure of the conformational order of surfactants at the interface. The appearance of the methyl modes as dips rather than peaks in Figure 2 shows that these groups are oriented toward the hydrophobic surface and away from the aqueous phase.18 It is possible to compare the structure of dodecanol with that of the trialkyl STC surfactants at the interface using the spectra in Figure 2. Figure 2a shows the SF spectrum of dodecanol. The absence of methylene resonances and the appearance of strong methyl resonances show that dodecanol forms a highly ordered monolayer at the surface. This structure is not surprising as dodecanol has a small uncharged head group permitting close packing of heads. The tri-chain spectra (Figure 2b-e), however, show less intense methyl resonances and pronounced methylene resonances. Hence, the tri-chains must form monolayers with less conformational order than dodecanol. All the available evidence suggests that the packing density of tri-chain overlayers is less than that of dodecanol.27,28 Thus high packing density leads to strong orientational order and low densities to poor order, as observed in previous studies for a variety of surfactants on this substrate.22 The fact that the intensity of the methylene resonance at 2855 cm-1 decreases from trihexyl STC to tributyl STC most likely reflects the different number of CH2 units in the molecules rather than changes in conformational order. Nevertheless the CH2 resonances are much more intense in the tri-chains than in dodecanol which contains more -(CH2)- groups. Figure 3 shows the SF spectra of pure trihexyl STC as a function of concentration. At the lower concentrations the methyl resonances are weaker, but the methylene resonances change very little with concentration. Such behavior indicates that the hydrocarbon chains become progressively more disordered as the solution concentration decreases. As discussed above increasing conformational disorder is connected with a decrease in surface packing density. A reduction in packing density is to be expected here since a decrease in the concentration of nonaggregated surfactant in solution decreases adsorption of that species.4,29,30 Tributyl STC and tripentyl STC gave results very similar to those of trihexyl STC.
Figure 3. Sum-frequency spectra of pure trihexyl STC as a function of concentration. Concentrations are expressed as a fraction of the cmc of trihexyl STC (1.78 × 10-4 M) (the spectra are offset vertically for clarity; zero intensity refers to 1/3 cmc).
A plausible explanation for the low conformational order of the tri-chain surfactants at the interface could be electrostatic repulsion between the head groups preventing close packing between the molecules, leading to a loosely packed, liquid like film. Electrostatic repulsion between head groups has been invoked to explain the high conformational disorder of ionic surfactants such as SDS at interfaces.22 More directly relevant, however, is that Aveyard et al.13 have shown the importance of head-group repulsions in tri-chain microemulsions by modifying their Winsor phase behavior with salt. The importance of electrostatic repulsion in this study was tested by taking the spectrum of trihexyl citrate, a surfactant which is structurally similar to trihexyl STC except that it contains an uncharged hydroxyl group instead of the charged sulfonate group of the carballylate (Figure 1b). The spectrum of the trihexyl STC (Figure 2d) was found to be almost identical to that of the trihexyl citrate (Figure 2e). The similarity implies that electrostatic or other head-group repulsion is not an essential factor for determining the order and packing density of the adsorbed tri-chain molecules. The low order of the three short-chain STCs is most probably due to the relatively weak (van der Waals) attractions between them. Mixed Surfactants. Before discussing spectra of mixed surfactant films, it is important to consider certain experimental issues. In particular, since low surfactant concentrations are used here, we need to consider whether adsorption is complete in the time allowed and whether adsorption in the cell significantly depletes the surfactant in solution. Two experiments were performed to investigate these points. First, spectra of mixed films (trihexyl STC with dodecanol) were recorded at 1/10 cmc every 3 h for 15 h. These spectra were identical with each other, confirming that equilibrium was reached within 30 min. Second, an SF spectrum of trihexyl STC at 1/3 cmc with dodecanol was taken after 15 mL of the solution was allowed to trickle through the cell over a period of 1 h, while the sample was regularly agitated to encourage adsorption. This spectrum was identical with that taken with the normal (static) method, and so depletion effects were insignificant.
Coadsorption of Tri-chain Surfactants and Dodecanol
Figure 4. Sum-frequency spectra in the C-H stretching region of mixed films as a function of tri-chain concentration. Filled circles represent spectra of trihexyl STC with d-C12OH, and open circles represent spectra of trihexyl STC with h-C12OH. Concentrations are expressed as a fraction of the cmc of trihexyl STC (1.78 × 10-4 M). Dodecanol was present in solution at 10-5 M (the spectra are offset vertically for clarity; zero intensity refers to 1/3 cmc).
Spectra obtained when mixed solutions of trihexyl STC and dodecanol were used are shown in Figure 4. The spectra (filled circles) of mixtures of tri-chain and perdeuterated dodecanol (d-C12OH) enable the resonances of the tri-chain to be seen in isolation. To appreciate how the adsorption of the tri-chain component is affected by the presence of the dodecanol, these spectra should be compared with those of the pure tri-chain
J. Phys. Chem., Vol. 100, No. 1, 1996 277 (Figure 3). Even though the trihexyl STC adsorbs strongly from its pure solution at 1/100 cmc, little or no trihexyl STC adsorbs in the presence of saturated dodecanol due to displacement of the tri-chain from the surface by the dodecanol. As the trihexyl STC concentration is raised, however, the tri-chain adsorbs to an increasing extent. At concentrations of 1/3 cmc and higher, the adsorption of the tri-hexyl STC in the presence of dodecanol is equivalent to that in its absence. The use of mixtures of tri-chain and unlabeled dodecanol (hC12OH) enables the adsorption of both components to be seen. The difference between the spectra with h-C12OH (open circles in Figure 4) and those with d-C12OH (closed circles) gives the signal from the dodecanol. At 1/100 cmc of the trihexyl STC, the spectrum of the mixed system is identical with that of pure dodecanol (Figure 2a), indicating a full monolayer of dodecanol on the surface. The intensity of the dodecanol signal, and hence dodecanol adsorption, decreases as the concentration of tri-chain is increased. This decrease mirrors the increase in the adsorption of tri-chain over the same range, described earlier. Above the cmc, there appears to be no dodecanol adsorption at all. Hence the uptake of dodecanol by the trihexyl STC micelles in solution probably competes so effectively with dodecanol adsorption on the substrate that dodecanol adsorption is minimal. Spectra from pure solutions of triethylbenzene STC and its mixtures with dodecanol are shown in Figure 5 for representative concentrations. Only spectra of the pure tri-chain and of its mixture with h-C12OH (and not d-C12OH) are shown since the resonances of h-C12OH do not obscure those of the aromatic tri-chain. For this tri-chain, and for the tributyl STC and tripentyl STC (not shown), the composition of the mixed films appears to have much the same dependence on solution concentration as it did with tri-hexyl STC. Dodecanol only adsorbs below the cmc and its coverage increases with decreasing tri-chain concentration while that of the tri-chain decreases. Discussion The adsorption behavior from mixtures of tri-chain and dodecanol can be interpreted as a competition between the two
Figure 5. Sum-frequency spectra of (a) pure triethylbenzene and (b) triethylbenzene STC with h-C12OH, as a function of concentration. Concentrations are expressed as a fraction of the cmc of triethylbenzene STC (2.40 × 10-3 M). The dodecanol was present in solution at 10-5 M (the spectra are offset vertically for clarity; zero intensity refers to 1/3 cmc).
278 J. Phys. Chem., Vol. 100, No. 1, 1996 species. For trihexyl STC concentrations below 1/10 cmc, dodecanol is adsorbed almost exclusively, whereas above the cmc only the tri-chain is adsorbed. However, in the region of 1/3 of the tri-chain cmc detailed fits to the spectra of trihexyl STC reveal an interesting subtlety. The spectrum of each trichain in the mixed film is identical with that in the film of pure tri-chain, implying that the tri-chains form a loosely packed layer. However, the fits also suggest that about 2/3 of a monolayer of dodecanol is present in the mixed film. Hence, instead of displacing tri-chain molecules in the overlayer, dodecanol is filling vacancies in the incomplete tri-chain monolayer. Using the literature values for the area per molecule of the tri-chain and dodecanol at the air-water interface of 12727 and 19 Å2,28 respectively, leads to the conclusion that there are approximately 7 dodecanol molecules to every tri-chain molecule in the mixed films at this concentration (1/3 cmc). The conformational order of each species in the mixed films of the tri-chain and dodecanol gives valuable information on the mixing at the interface. This order can be gauged from the intensity ratio of its CH3/CH2 sum-frequency resonances (r+/ d+). Fits to the spectra of tributyl STC and tripentyl STC (not shown), and trihexyl STC (Figure 3) at different concentrations revealed no significant change in r+/d+ for any of these species in the mixed films and the same species in the pure films. In all these mixed films the dodecanol remains more ordered than the tri-chains. Had there been extensive mixing between dodecanol and the aliphatic tri-chains, their conformational order would have become more similar. The situation is somewhat different in the case of triethylbenzene STC, however. The r+/ d+ ratio of dodecanol is significantly reduced from the values in pure dodecanol films when mixed with this tri-chain. It appears that the bulky aromatic groups of triethylbenzene STC disrupt the high conformational order of the coadsorbed dodecanol, whereas the aliphatic groups of the tri-n-alkyl STC molecules do not. It is interesting to compare the mixed films that dodecanol forms with tri-chains with those it forms with SDS. Two pieces of experimental evidence suggest that the degree of mixing between different species in films of tri-n-alkyl STC/dodecanol is low in comparison with films of SDS/dodecanol. First, the SFS results show that the conformational order of SDS and of tri-chains in these mixed films are different despite the fact that they are similar in the pure films (both less ordered than dodecanol). The order of SDS and dodecanol molecules become equivalent upon coadsorption,31 implying that their alkyl chains must be mixing extensively. In contrast, the results here show that the order of the tri-chain molecules remains much lower than that of dodecanol, suggesting minimal mixing of their chains. Second, when traces of dodecanol are present in dilute solutions of SDS the adsorption of the latter is enhanced at the air-water interface32 in contrast to the dodecanol/tri-chain results where coadsorption occurs only over a small range of compositions. The difference in the behavior of the two systems probably reflects the differences in the structures of SDS and the tri-chains. The chain lengths of SDS and dodecanol molecules are the same, and they are both single chain species. It is therefore reasonable to expect them to have favorable mutual (van der Waals) interactions along the lengths of their chains leading to facile mixing and favorable coadsorption. Trichain molecules are shorter and have a markedly different structure to dodecanol, resulting in weak attractions to and hence poor coadsorption with molecules of dodecanol on the surface. The results for the overlayers formed with tri-chains present in solutions at 1/3 cmc suggest that dodecanol does not perturb the tri-chain molecules at the interface but merely occupies
Johal et al. vacancies around them. Because the number density of the dodecanol exceeds that of the tri-chain in the ratio 7:1, these vacancies must be quite large and may be interlinked by dodecanol. The presence of extended regions of dodecanol would explain why the conformational order of the dodecanol is the same when mixed with the tri-n-alkyl STC as in the pure monolayer of dodecanol. The tri-chain molecules may also cluster together as small aggregates.33 Large aggregates, however, are inconsistent with the observation that coadsorption occurs over a range of solution compositions. Macroscopic islands of tri-chain and dodecanol would derive negligible stabilization from any mixing at their edges. They would only be stable at the unique composition where the free energies of adsorption of the islands are equivalent. At other compositions only one type of island or the other would be thermodynamically stable at the surface. The model of coadsorption which best fits the data is one in which dodecanol forms a “sea” of ordered molecules within which lie monomers or small clusters of disordered tri-chain molecules. Conclusion Sum-frequency spectroscopy has been used to study the coadsorption of tri-chain surfactants and dodecanol at the solidliquid interface. Pure dodecanol produces a densely packed monolayer at a hydrophobic surface, whereas pure tri-chain surfactants yield disordered monolayers. The disordering is not due to electrostatic repulsion between the head groups but arises because the short alkyl chains are only weakly attracted by van der Waals forces. Upon mixing there is competition between the adsorption of tri-n-alkyl STC and dodecanol, with coadsorption occurring over a small range of tri-chain concentrations. Above the cmc, dodecanol dissolves in micelles and only the tri-chain adsorbs at the solid surface. Molecules of tri-n-alkyl STC and dodecanol coadsorb with the same structure as in their pure films. The absence of structural changes implies that there is minimal mixing between the alkyl chains of the dodecanol and tri-chain molecules. However, when the tri-chains have bulky ethylbenzene tail groups, the adsorbed dodecanol becomes disordered. Acknowledgment. We thank Kodak Ltd. and the EPSRC for a CASE studentship for M.S.J. and Unilever Research, Port Sunlight Laboratory for support. We would also like to thank Dr. A. Pitt (Kodak Ltd.) for supplying the tri-chain surfactants, Dr. R. K. Thomas (Oxford University) for the gift of the perdeuterated dodecanol, and Mr. A. M. Briggs (University of Cambridge) for experimental assistance. References and Notes (1) Moroi, Y.; Nishikido, N.; Matsoura, R. J. Colloid Interface Sci. 1977, 61, 233. (2) Tokuka, Y.; Uchiyama, H.; Abe, M. J. Phys. Chem. 1994, 98, 6167. (3) Somasunduran, P.; Fu, E.; Qun Xu Langmuir 1992, 8, 1065. (4) Bain, C. D.; Davies, P. B.; Ward, R. N. Langmuir 1994, 10, 2060. (5) Miles, G. D. J. Chem. Phys. 1945, 49, 71. (6) Nilsson, G. J. Phys. Chem. 1957, 61, 1135. (7) Penfold, J.; Thomas, R. K.; Simister, E.; Lee, E. M.; Rennie, A. J. Phys. Condens. Matter 1990, 2, SA411. (8) Esumi, K.; Nagahama, T. Colloids Surf. 1989, 36, 353. (9) So¨derlind, E.; Stilbs, P. Langmuir 1993, 9, 1678. (10) Lu, J. R.; Hromadoua, M.; Simister, E. A.; Thomas, R. K. J. Phys. Chem. 1994, 98, 11519. (11) McDermott, D. C.; Lu, J. R.; Lee, E. M.; Thomas, R. K.; Rennie, A. R. Langmuir 1992, 8, 1204. (12) Lee, L. T.; Guiselin, O.; Farnoux, B.; Lapp, A. Macromolecules 1991, 24, 2518. (13) Aveyard, R.; Binks, B. P.; Clark, S.; Fletcher, P. D.; Giddings, H.; Kingston, P. A.; Pitt, A. Colloids Surf. 1991, 59, 97. (14) Shen, Y. R. Nature 1989, 337, 519.
Coadsorption of Tri-chain Surfactants and Dodecanol (15) Eisenthal, K. B. Annu. ReV. Phys. Chem. 1992, 43, 627. (16) Bain, C. D.; Davies, P. B.; Ong, T. H.; Ward, R. N.; Brown, M. A. Langmuir 1991, 7, 1563. (17) Shen, Y. R. Principles of Nonlinear Optics; Wiley: New York, 1984. (18) Ward, R. N.; Davies, P. B.; Bain, C. D. J. Phys. Chem. 1993, 97, 7141. (19) Linfield, W. M. Anionic Surfactants, Surfactant Science Series; Marcel Dekker: New York, 1976; Vol. 7, Chapter 12. (20) Moreno, J. M. M.; Ronquero, A. V.; Barroso, R. C.; Torregrosso, R. E.; Cruz, J. R. Grasas Aceites 1961, 12, 267. (21) Nawiasky, P. US Patent 2,315,375, 1943, to General Aniline and Film Corp. (22) Ward, R. N.; Duffy, D. C.; Davies, P. B.; Bain, C. D. J. Phys. Chem. 1994, 98, 8536. (23) Ong, T. H.; Davies, P. B.; Bain, C. D. Langmuir 1993, 9, 1836. (24) Dubois, L. H.; Nuzzo, R. G. Annu. ReV. Phys, Chem. 1992, 43, 437.
J. Phys. Chem., Vol. 100, No. 1, 1996 279 (25) Varrsanyi, G. Vibrational Spectra of Benzene DeriVatiVes; Academic Press: New York, 1969. Duffy, D. C.; Davies, P. B.; Bain, C. D. J. Phys. Chem. 1995, 99, 15241. (26) Fuson, N.; Garriguo-Lagrange, C.; Josien, M. L. Spectrochim. Acta 1960, 16, 100. (27) Pitt, A., Kodak Ltd., private communication. (28) Legrand, J. F.; Renault, A; Konovalov, O; Chevigny, E.; Alsnieisen, J. Grubel, G.; Berge, B. Thin Solid Films 1994, 248, no. 1, 95. (29) Yeo, J.; Stauss, G. Langmuir 1992, 8, 2277. (30) Messmer, M. C.; Conboy, J. C.; Richmond, G. L. J. Am. Chem. Soc. 1995, 117, 8039. (31) Ward, R. N.; Johal, M. S.; Davies, P. B.; Bain, C. D., manuscript in preparation. (32) Lu, J. R.; Purcell, I. P.; Lee, E. M.; Simister, E. A.; Thomas, R. K.; Rennie, A.R.; Penfold, J. J. Colloid Interface Sci., submitted. (33) Israelachvili, J. Langmuir 1994, 10, 3774.
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